Metal surface treating process by use of lanthanum compounds

ABSTRACT

METHOD FOR IMPROVING THE OXIDATION RESISTANCE OF IRON, NICKEL AND COBALT BASE ALLOYS CONTAINING CHROMIUM BY CONTACTING AN ALLOY OF THIS TYPE WITH A HEAT DECOMPOSABLE LANTHANUM COMPOUND AT ELEVATED TEMPERATURE.

United States Patent O 3,592,681 METAL SURFACE TREATING PROCESS BY USEOF LANTHANUM COMPOUNDS Henri Hatwell and Willy Robert De Sutter,Brussels, Belgium, assignors to Cabot Corporation N Drawing. Filed May15, 1968, Ser. No. 729,400 lint. Cl. C230 3/ 04, 9/00 US. Cl. 117-50 13Claims ABSTRACT OF THE DISCLOSURE Method for improving the oxidationresistance of iron, nickel and cobalt base alloys containing chromium bycontacting an alloy of this type with a heat decomposable lanthanumcompound at elevated temperature.

BACKGROUND OF THE INVENTION This invention relates to the treatment ofmetal surfaces to improve the oxidation resistance thereof. Moreparticularly, the present invention is directed to a method for treatingthe surfaces of chromium-containing cobalt, nickel and iron base alloyswith lanthanum compounds to improve the oxidation resistance of suchalloys without significantly adversely affecting the mechanicalproperties of the alloys.

Cobalt, nickel and iron base alloys have been widely used in the past inhigh temperature applications, such as for example, turbine blades whichrequire high strength at temperature, and these alloys have been foundto be, in general, highly effective. However, it is very important, inmany high temperature situations, such as that mentioned above, that thematerials employed be also highly resistant to oxidation.

To meet this requirement, it has been proposed to incorporate lanthanumin certain molten alloys whereby the resulting cast or wrought materialis improved with respect to oxidation resistance. However, depending onthe alloy composition, alloyed lanthanum can lead to the formation ofintermetallic phases which can adversely affect the forgeability of thematerial.

It is therefore an object of the present invention to provide a methodfor improving the oxidation resistance of cobalt, nickel and iron basealloys Without significantly adversely aifecting the mechanicalproperties of the alloys.

Other objects will be apparent from the following description andclaims.

A method in accordance with the present invention for improving theoxidation resistance of cobalt, nickel and iron base alloys comprisescontacting a substrate of cobalt, nickel or iron base alloy containingfrom about 15 to 25% chromium with lanthanum oxide or a heatdecomposable lanthanum salt at a temperature sufiicient to causelanthanum to be incorporated in the alloy substrate.

By the practice of the present invention, a wide range of cobalt, nickeland iron base alloys can be improved with respect to oxidationresistance. Such alloys contain, in addition to Co, Ni and Fe, and fromabout 1525% Cr up to 15% W, and may contain minor proportions ofelements ccustomarily present in Co, Ni and Fe base alloys. Thefollowing Table I shows specific alloys which can be treated by thepractice of the present invention.

Table I 22% Cr, up to 1% W, 18% Fe, up to 1% Si, 2% Co,

9% Mo, up to 1% Mn, up to 0.15% Cr, bal. Ni.

20% Cr, 15% W, up to 3% Fe, up to 1% Si, 10% Ni,

1.5% Mn, up to 0.15% C, bal. Co.

21% Cr, 14% W, 2% Fe, 22% Ni, 0.1% C, 0.3% Si,

bal. Co.

In addition to lanthanum oxide, lanthanum salts which are decomposableby heating at temperatures below the melting point of the alloy to betreated, to provide a gaseous reaction product can also be used in thepractice of the present invention. A preferred salt is lanthanumnitrate, La(NO .6H O which can be employed in the practice of thepresent invention in the form of a sprayquench solution as hereaftermore particularly illustrated. Other useable soluble lanthanum saltswhich can be employed in the form of a spray or quench solution arelanthanum acetate, lanthanum bromate, lanthanum bromide, lanthanumchloride, lanthanum iodate, lanthanum iodide and lanthanum sulfate.Insoluble lanthanum salts such as lanthanum carbonate, lanthanummolybdate and lanthanum oxalate can be used in the form of slurries inthe practice of the present invention as hereinafter described.

In the case of lanthanum oxide, this material is used in the form of aslurry of La(OH) In a particular embodiment of the present invention, anaqueous solution of lanthanum salt is prepared, for example, bydissolving La(NO .6H O in water. The alloy material to be treated,suitably in strip form, is heated in an inert environment to atemperature sufficient to decompose the lanthanum salt, e.g. about 1100C. and then plunged into and quenched in the lanthanum salt solution.

The resulting material, in which lanthanum is incorporated as a resultof the aforedescribed treatment, exhibits a remarkably increasedresistance to oxidation.

In the embodiment as aforedescribed involving the quenching of an alloysubstrate in a lanthanum salt solution, the preferred temperature rangefor the heated metal substrate is from about 300 C. to 1200 C. and thelanthanum concentration in the solution is suitably at least about 2%.Treatment of alloy substrates in the foregoing range provides for theincorporation of at least about 0.4 mg./cm. of lanthanum, and thelanthanum content can range up to 2 mg./cm. This lanthanum concentrationensures improved resistance to oxidation.

In a further embodiment of the present invention, a solution oflanthanum salt in the form of fine droplets is sprayed onto a heatedcobalt base alloy, in an inert gas environment, whereby the lanthanumsalt is decomposed and lanthanum is incorporated in the alloy substrate.

In this embodiment, salt solutions having the concentrations previouslynoted can be used and similar substrate temperatures are employed toproduce as oxidation resistant material. Atomizer type apparatus can beused to apply the lanthanum salt solution to the heated alloy substrateand it has been found that increased amounts of lanthanum can beincorporated in the alloy substrate by spraying in this manner ascompared to the quench technique previously described.

In the spray embodiment of this invention by spraying between about 0.4to 2 milligrams of lanthanum per square cm. of substrate surface, a veryhighly oxidation resistant material is obtained.

In a still further embodiment of the present invention, a slurrycontaining heat decomposable lanthanum salt is applied, e.g. painted onthe cobalt base alloy substrate to be heated and the thus prepared alloyis heated to an elevated temperature sufficient to decompose thelanthanum salt and cause lanthanum to be incorporated in the cobalt basealloy substrate.

By way of example, the slurry can be in the form of lanthanum hydroxideor a lanthanum nitrate solution with a suitable wetting agent added andthe temperature range in which the slurry coated substrate is heated issuitably in the range of about 300* to 1200 C.

The following examples will further illustrate the present invention.

3 EXAMPLE I Wrought sheet samples 20 x 15 X 1.5 mm. were prepared fromthe following composition:

Cr Ni W Fe Si M La 00 A 20 20 Bal B 20 20 01 Bal A solution was preparedby dissolving 20 grams of La(NO .6H O in 50 grams of water. The sampleswere mechanically polished with 600 grit abrasive paper and thenelectrolytically polished. Each sample was then heated to 1100 C., andquenched; sample A was quenched in the prepared lanthanum salt solutionand sample B was quenched in water. The heating and quenching 'wasperformed under argon atmosphere. After quenching the samples wererubbed lightly with filter paper and dried.

The weight gain of the samples quenched in the lanthanum solution was inthe order of 0.4 milligram per cm.

The samples were then subjected to isothermal oxidation tests byexposure to dry air at 1000 C. for fifty hours. A sample A having thesame composition as sample A, but which was not quenched in thelanthanum salt Was also tested for comparative purposes. The results areshown below:

Sample Reacted Spelled weight oxygen, oxide, change,

Sample lug/em. lug/cm. 1ng./em.

A: No La treatment 2. 5 0.4 2. 2 A: La quenched 0.14 0.11 0.08 B: No Latreatment 0. 6 0.03 0. 56

The results of Example I show the improved oxidation resistance ofalloys treated in accordance with the present invention as compared tountreated alloys and alloys containing lanthanum as an alloyingconstituent.

EXAMPLE II Samples were prepared as in Example I and a sample having thesame composition A was placed in a chamber filled with argon andinduction-heated to 900 C. Using an atomizer assembly, the lanthanumsalt solution of Example I was sprayed on the samples. The temperatureof the sample was increased to 1000-1050 C. and maintained in this rangefor two minutes after which the samles were permitted to cool to roomtemperature.

After being cooled, the sample was placed in boiling distilled water for10 minutes and then rubbed with moist I filter paper and dried. Theweight grain of the sample was on the order of 0.4 to 2 milligrams percm.

The thus treated sample and untreated samples of compositions A and Bwere subjected to a cyclic oxidation test which involved minutes heatingin a furnace to 1000 C. and 10 minutes out of the furnace during whichthe sample was cooled to about 50 C. in air. The test was continued for100 hours, or 200 cycles. The results are shown in the table below:

Sample: Metal consumption mg./cm. ANo La treatment 66 BNo La treatment61 A-La sprayed 9 A-La sprayed 12 200 hour data.

An interesting and important feature of the present invention is thatthe method of the present invention does not provide a lanthanum coatingas such but a penetration of lanthanum into the cobalt base alloysubstrate which becomes more pronounced upon exposure to an oxidizingatmosphere at elevated temperature. Thus, by further treating thematerial prepared as hereinabove described, to air or oxygen at atemperature of 1000 C. or above, the surface of the material becomesessentially free of lanthanum although the amount of lanthanumincorporated in the alloy substrate is not significantly decreased. As aresult, exposure of the treated material to erosive environments doesnot significantly effect the long term oxidation resistance of thematerial as would be the situation in the case of a lanthanum coating orplating.

EXAMPLE III Alloy samples having the same composition as alloy A ofExample I were treated with lanthanum solution following the procedureof Example II and then rolled to reduction at 1000 C. Specimens 20 x 51x 2 mm. of the rolled alloy were cycled at 1000 C. for hours asdescribed in Example II. The metal loss was 18 mg./ cm. For untreatedspecimens the metal loss averaged 50 mg./cm.

EXAMPLE IV The procedure of Example III was followed except that thetreated alloy was cold rolled 45%. The metal loss of the treatedspecimens averaged 14 mg./cm. For untreated specimens the metal lossaveraged 38 mg./cm.

EXAMPLE V A sample of essentially 100% nickel was treated with lanthanumsolution following the procedure of Example II and cycled at 1000 C. for48 hours. The metal loss averaged 37.5 mg./cm.

What is claimed is:

1. A method for improving the oxidation resistant properties of alloysselected from the group consisting of cobalt, nickel and iron basealloys containing from about 15% to 25% chromium which comprisescontacting a heated substrate of the selected alloy with a liquiddispersion of a material selected from the group consisting of lanthanumoxide and heat decomposable lanthanum salts at a temperature sufiicientto cause lanthanum to be incorporated into the alloy substrate.

2. A method in accordance with claim 1 wherein the temperature isbetween about 300 and 1200 C.

3. A method in accordance with claim 1 wherein lanthanum is incorporatedin the alloy substrate in an amount between about 0.4 and 2 mg./cm.

4. A method in accordance with claim 1 wherein the alloy substrate isheated to a temperature sufficient to decompose the selected lanthanumsalt and is immersed in a solution of the selected lanthanum salt.

5. A method in accordance with claim 1 wherein the selected alloysubstrate at a temperature from about 300 to 1200 C. is immersed in asolution of lanthanum salt.

6. A method in accordance with claim 5 wherein the concentration oflanthaum in the solution is at least about 2%.

7. A method in accordance with claim 1 wherein a solution of lanthanumsalt is sprayed onto the selected alloy substrate, said substrate beingat an elevated temperature sufficient to decompose the lanthanum salt.,

8. A method in accordance with claim 7 wherein the alloy substrate is ata temperature between about 300 and 1200 C.

9. A method in accordance with claim 7 wherein the lanthanum content ofthe spray is at least about 2%.

10. A method in accordance with claim 7 wherein between about 0.4 and 2milligrams per cm. of lanthanum are sprayed onto a cobalt base alloysubstrate.

11. A method in accordance with claim 1 wherein the alloy substrate iscontacted with a slurry of heat decomposable lanthanum salt.

12. A method in accordance with claim 11 wherein the alloy substrate isheated to a temperature between about 300 and 1200 C.

13. A method in accordance with calim 12 wherein at least about 0.4milligrams per cm. of lanthanum deposited on the alloy substrate.

FOREIGN PATENTS 12/1957 Great Britian.

RALPH S. KENDALL, Primary Examiner US. Cl. X.R.

